#define _CRT_SECURE_NO_WARNINGS
#include<stdio.h>
int main()
{
	int arr[] = { 1,2,3,4,5,4,3,2,1 };
	int i = 0;
	int len = sizeof(arr)/sizeof(arr[0]);
	int s = 0;
	for (i = 0;i < len;i++)
	{
		s ^= arr[i];
	}
	printf("%d", s);
	return 0;
}
